1. Field of the Invention
The present invention relates to a fabrication method of a Liquid Crystal Display (LCD) device, and more particularly, to a fabrication method of a liquid crystal display device in which a pattern is formed by using a printing method.
2. Description of the Related Art
Recently, various portable electric devices, such as mobile phones, Personal Data Assistants (PDA), and notebook computers, have been developed. Such devices need a flat panel display device that is small, light weight, and power-efficient. To meet such needs, flat panel display device technologies, such as Liquid Crystal Display (LCD) technology, Plasma Display Panel (PDP) technology, Field Emission Display (FED) technology, and Vacuum Fluorescent Display (VFD) technology have been actively researched and developed. Of these flat panel display devices, the LCD is most prevalent because it can be mass produced, can be efficiently driven, and has a superior image quality.
FIG. 1 is a plan view showing a related art LCD that uses a thin film transistor as an active device. As shown in FIG. 1, each pixel includes a Thin Film Transistor (TFT) T formed adjacent to where a gate line 4 and a data line 6 cross one another in a pixel 1 of a display region. The gate line 4 carries control signals from an external driving circuit. The data line 6 carries an image signal. The TFT includes a gate electrode 3 connected to the gate line 4; a semiconductor layer 8a formed on the gate electrode 3; and source electrode 5a and drain electrode 5b formed to connect to the semiconductor layer 8a. The TFT is activated in response to a control signal applied to the gate electrode 3 from the gate line 4. A pixel electrode 10 is connected to the drain electrode 5b. The liquid crystal (not shown) in the pixel 1 is driven by applying an image signal through the source electrode 5a and drain electrode 5b when the semiconductor layer 8a is activated in response to a control signal applied to the gate electrode 3.
FIG. 2 is a cross sectional view of the related art TFT positioned in each pixel. As shown in FIG. 2, the TFT includes a substrate 10a made of transparent insulating material, such as glass. A gate electrode 3 is formed on the substrate 10a. A gate insulating layer 2 is formed over the entire substrate 10a on which the gate electrode 3 is formed. A semiconductor layer 8a is formed on the gate insulating layer 2. The source electrode 5a and drain electrode 5b are formed over the semiconductor layer 8a and respectively connected to the semiconductor layer 8a by ohmic semiconductor layers 8b. A pixel electrode 10 is connected to the drain electrode 5b. A passivation layer 9 is formed on the source electrode 5a and drain electrode 5b for protecting the device. The source and drain electrodes 5a and 5b of the TFT are electrically connected to a pixel electrode 10 when the TFT is activated to display an image in accordance with the signal applied to the pixel electrode 10 through the source electrode 5a and drain electrode 5b. 
The TFT is fabricated with a manufacturing method that uses several different mask processes. The material and time consumed in each mask process directly impacts productivity. Thus, reducing the number of mask processes from a five mask process down to a four mask process increases productivity. A method for fabricating a TFT in accordance with a related art four mask process will be explained with reference to FIGS. 3A to 3E that sequentially shows a related art four mask process to manufacture a TFT.
As shown in FIG. 3A, a metallic layer is deposited on a transparent substrate 20, and then a first photoresist pattern 23a is formed by a photolithography process. A gate electrode 23 is then formed using the resist pattern 23a as a mask. The photolithography process includes depositing the photoresist, exposing the photoresist, developing the photoresist, and etching away the undesired portion of material to be patterned. During the time that the photoresist is being exposed, a mask over the photoresist is used to define a resist pattern. Also, in the etching process, a metal pattern, such as a gate electrode, is substantially formed by using the resist pattern formed after the development process as a mask. Subsequently, the resist pattern that remains on the metal layer is removed.
As shown in FIG. 3B, inorganic material, such as SiNx or SiOx, a semiconductor layer 28a and 28b, and a metallic layer 25 are sequentially deposited on the substrate 20 on which the gate electrode 23 is formed. Then, a second resist pattern 23b, which remains over a channel region is formed with a photolithography process by using a second mask (not shown). A diffraction exposure is applied to the photoresist layer on the metallic layer 25 over the gate electrode 23 such that a portion of the resist pattern over the center of the channel region has a thinner thickness than the other regions of the resist pattern. Then, etching is performed using the second resist pattern 23b as a mask until the gate insulating layer 22 around the TFT is exposed. Next, as shown in FIG. 3C, the resist pattern region to which the diffraction exposure is applied is removed to form a third resist pattern 23c that is used to expose the metallic layer 25. Then, the metallic layer 25 is etched by using the third resist pattern 23c as a mask to thereby form source electrode 25a, drain electrode 25b, and ohmic semiconductor contact layers 28b. 
The second mask described above is a diffraction mask that has different light transmittance ratios, so that a thickness of the resist pattern can be different in different portions of the resist pattern. Generally, a diffraction mask is used to pattern a photoresist that can be etched at least twice so that the same photoresist can be used to do at least two different patterns. For example, a single mask can be used to pattern active layers 28a and 28b, and metal layer 25 can be patterned, and then another patterning is performed to form the source electrode 25a, drain electrode 25b and ohmic semiconductor contact layers 28b. Thus, a five mask process for forming a TFT can be reduced to a four mask process.
As shown in FIG. 3D, the resist pattern which remains on the source electrode 25a and drain electrode 25b is removed. An inorganic material, such as SiOx or SiNx, or an organic material, such as BCB (benzocyclobuten) or acryl, is deposited as a passivation layer 29 over the entire device including the source electrode 25a and drain electrode 25b. A fourth resist pattern 23d is formed using a third mask with a photolithography process. A hole is opened in the passivation layer 29 to exposes a part of the drain electrode 25b using the third mask. Then, as shown in FIG. 3E, the fourth resist pattern is removed, and a transparent conductive material, such as ITO (Indium Tin Oxide) is deposited on the passivation layer 29 and patterned by using a fourth mask to form a pixel electrode 31.
The photolithography process for fabricating the TFT is performed by repeatedly performing a series of consecutive processes, such as deposition, alignment, exposure, development, etching/deposition/implantation and cleaning. More specifically, the series of processes includes depositing the photoresist, aligning the mask to an alignment key, developing the photoresist by irradiating a light source on exposed portions of the photoresist, removing undeveloped resist, performing the etching/deposition/implantation manufacturing step and then cleaning the remaining resist from the device. As the number of masking processes increases, the greater chances are that misalignment will occur. Aligning the mask on the substrate, especially with sophisticated patterns, requires high accuracy. Otherwise, a major defect can occur. Thus, misalignment of the mask can also occur regardless of the number of masking processes.
Also, depositing the photoresist uniformly on the substrate includes: a pre-bake for improving adhesion of the photoresist by removing moisture from a surface of the substrate on which the photoresist will be deposited; spin-coating the photoresist onto the surface of the substrate by using a centrifugal force; and a soft-bake for solidifying the photoresist by evaporating solvent from the deposited photoresist. Spin coating is a process for distributing photoresist by a centrifugal force of the substrate in which photoresist is dropped into the center of a rotating substrate. The advantages of spin coating photoresist are that it is simple and fast. However, the disadvantages is that very little of the photoresist is deposited on the surface of the substrate. Most of the photoresist is wasted, which increases material cost. Further, the equipment needed for deposition, alignment, exposure, and cleaning are expensive, which also increases production cost.